The disclosed technology relates to lubricants with good oxidative stability, containing a combination of an oil-soluble titanium compound and a condensation product of a carboxylic-functionalized polymer with an aromatic moiety.
Engine manufacturers have focused on improving engine design in order to minimize emissions of particulates, emissions of other pollutants, and also improve cleanliness, fuel economy, and efficiency. One of the improvements in engine design is the use of exhaust gas recirculation (EGR) engines. While improvements in engine design and operation have contributed to reducing emissions, some engine design advances are believed to have generated other challenges for the lubricant. For example, EGR is believed to have led to increased formation and/or accumulation of soot and sludge.
Soot-mediated oil thickening is common in heavy duty diesel engines. Some diesel engines employ EGR. The soot formed in an EGR engine has different structures and causes increased viscosity of engine lubricant at lower soot levels than soot formed in an engine without an EGR. Attempts to alleviate soot-mediated oil thickening have included the use of dispersants containing aromatic functionality, as disclosed, for instance, in PCT publication WO2010/062842, Jun. 3, 2010; US Application 2006-0025316, Covitch et al., Feb. 2, 2006; or US Application 2006-0189492, Bera et al., Aug. 24, 2006.
At the same time, there has been continuing interest in providing crankcase lubricants meeting government specifications such as reduced sulfur and phosphorus limits. It is widely believed that lowering these limits may have a serious impact on engine performance, engine wear, and oxidation of engine oils. This is because historically a major contributor to phosphorus content in engine oils has been zinc dialkyldithiophosphate (ZDP), and ZDP has long been used to impart antiwear and antioxidancy performance to engine oils. Thus, as reduced amounts of ZDP are anticipated in engine oils, there is a need for alternatives to impart protection against deterioration in one or more of the properties of engine performance, engine wear, and oxidation of engine oils. Such improved protection is desirable whether or not ZDP and related materials are included in the lubricant. Lubricants addressing these issues have been formulated containing oil-soluble titanium compounds, as disclosed in U.S. Pat. No. 7,727,943, Brown et al., Jun. 1, 2010, and U.S. Pat. No. 7,615,520, Esche, J R, Nov. 10, 2009.
There remains a need, however, for lubricants having good or improved properties such as described above, including further improved oxidative stability. The technology disclosed herein provides a lubricant having one or more of such properties.